Vascular endothelial cells and blood platelets play key roles in a number of biological responses by selectively binding certain cells, for instance phagocytic leukocytes, in the blood stream. For example, endothelial cells preferentially bind monocytes and granulocytes prior to their migration through the blood vessel wall and into surrounding tissue in an inflammatory response. Certain inflammation-triggering compounds are known to act directly on the vascular endothelium to promote the adhesion of leukocytes to vessel walls. Cells then move through the walls and into areas of injury or infection. Cellular adhesion to vascular endothelium is also thought to be involved in tumor metastasis. Circulating cancer cells apparently take advantage of the body's normal inflammatory mechanisms and bind to areas of blood vessel walls where the endothelium is activated.
Blood platelets are also involved in similar responses. Platelets are known to become activated during the initiation of hemostasis and undergo major morphological, biochemical, and functional changes (e.g., rapid granule exocytosis, or degranulation), in which the platelet alpha granule membrane fuses with the external plasma membrane. As a result, new cell surface proteins become expressed that confer on the activated platelet new functions, such as the ability to bind both other activated platelets and other cells. Activated platelets are recruited into growing thrombi, or are cleared rapidly from the blood circulation. Activated platelets are known to bind to phagocytic leukocytes, including monocytes and neutrophils. Examples of pathological and other biological processes which are thought to be mediated by this process include atherosclerosis, blood clotting and inflammation.
Recent work has revealed that specialized cell surface receptors on endothelial cells and platelets, designated endothelial leukocyte adhesion molecule-1 (ELAM-1, E-Selectin) and granule membrane protein-140 (GMP-140, P-Selectin), respectively, are involved in the recognition of various circulating cells by the endothelium and platelets. These receptors are surface glycoproteins with a lectin-like domain, a region with homology to epidermal growth factor, and a region with homology to complement regulatory proteins (see, Bevilacqua et al., Science 243:1160 (1989), which is incorporated herein by reference). For example, E-Selectin has been shown to mediate endothelial leukocyte adhesion, which is the first step in many inflammatory responses. Specifically, E-Selectin binds human neutrophils, monocytes, eosinophils, certain T-lymphocytes (N. Graber et al., J. Immunol., 145:819 (1990)), NK cells, and the promyelocytic cell line HL-60.
The term "selectin" has been suggested for a general class of receptors, which includes E-Selectin and P-Selectin because of their lectin-like domain and the selective nature of their adhesive functions. These cell surface receptors are expressed on a variety of cells. P-Selectin (also known as PADGEM) is present on the surface of platelets and endothelial cells, where it mediates platelet-leukocyte and endothelium-leukocyte interactions. Another member of the selectin class is the MEL-14 antigen, and its human analog LAM-1, which are cell surface receptors of lymphocytes, and act as lymph node homing receptors. The exact nature of the ligand recognized by selectin receptors remains unknown.
Various other methods have been previously developed to block the action of selectins and thus inhibit cellular adhesion. For instance, the use of monoclonal antibodies directed to E-Selectin has been proposed as a method to inhibit endothelial-leukocyte adhesion as a treatment for pathological responses, such as inflammation. Endothelial interleukin-8 has also been shown to be an inhibitor of leukocyte-endothelial interactions.
With the elucidation of the ligand-receptor interaction, it will be possible to develop highly specific, efficient inhibitors of selectin-mediated cellular adhesion which would be useful in therapeutic regimens. The ligand(s) could also be used to target other pharmaceutical compounds, such as anti-inflammatory agents or anti-oxidants, to the sites of injury. To date, insufficient understanding of the interaction of the ligand(s) and receptor molecules on the respective cells has hindered these efforts. The present invention fulfills these and other related needs.